Toothbrushes are typically manufactured using an injection molding process. Such an injection molding process is characterized by providing a mold in the shape of the toothbrush and injecting molten plastic through a hot channel nozzle into the mold. The toothbrush is then cooled and ejected from the mold. For example, U.S. Pat. No. 5,845,358 shows such a toothbrush made by injection molding. This injection molding process may comprise a single injection molding step for a toothbrush made from a single component, or it may also comprise two or more injection molding steps for toothbrushes made from two or more components or materials. The second or subsequent materials may be of a significantly softer durometer than the first material to increase grip or provide distinctive design elements. Such an injection molding process is characterized by providing a mold in the shape of a portion of the toothbrush and injecting a first molten plastic through a hot channel nozzle into the mold, waiting some time for the part to sufficiently cool, then transferring the solid or semi-solidified part to a second molding cavity where a second material is injected into the void formed by the combination of the second mold and some surfaces of the first molded part. The toothbrush is then cooled and ejected from the mold. For example, U.S. Pat. No. 6,276,019 shows such a toothbrush made by injection molding. One of the limitations of the conventional injection molding processes is that large diameter handles cannot be produced in an efficient manner, due to the cost of increased material and lengthened cooling times, resulting from the increased mass of material used.
Toothbrushes with increased handle diameters provide substantial advantages, for instance they can provide increased gripping area for children, increasing the ability of children to handle and use toothbrushes; also people with disabilities such as arthritis sometimes have difficulty in handling toothbrushes due to difficulty in flexing the joints in their hands. Such difficulties are considerably relieved by means of toothbrushes having increased handle diameters. Additionally, the larger cross section handles on the toothbrushes are better for the user from an ergonomic point of view.
Toothbrushes with high-friction and/or low-durometer regions of a second material on the outer surface also provide substantial advantages in gripping. Low-durometer materials, such as those materials whose hardness is measured at less than approximately 90 on the Shore A scale, provide advantages in grip by deforming under the range of comfortable gripping forces. The deformation assists in holding the brush uniformly in position in the hand, and also provides a pleasing tactile feedback. Addition of high-friction grip surfaces directly reduces the squeezing force necessary to maintain a stable orientation of the brush bristles during use. Due to their low strength, however, toothbrushes made entirely from high-friction, low-durometer material are unlikely to exhibit the bend strength necessary to provide adequate force to brush in a conventional grip style. Thermoplastic Elastomers (TPEs) in the hardness range of Shore A 20-90 are a common second, third or subsequent material used to improve grip on toothbrushes and other personal care articles.
Variations in cross sectional area, including both larger and smaller cross sectional areas, along the length or major axis of the brush assist the user in the grip and handling of the brush during use, when it must be rapidly moved, often while wet or slippery. Additionally, materials that maintain a higher coefficient of friction when wet, including TPEs in the above-mentioned hardness range can assist in wet-grip situations.
Even though there are advantages to toothbrushes having increased handle diameters the use of injection molding to manufacture toothbrushes with larger cross section handles has at least six disadvantages:
First—the toothbrush is more expensive due to the use of more plastic to make the toothbrush. The material used to create the toothbrush handle increases approximately with the square of the diameter of the handle.
Second—the cost of manufacture is increased because the time needed to cool and solidify the toothbrush increases considerably. The increased cooling time is due both to the increased quantity of hot plastic, and the larger cross section of the toothbrush. As plastic has a relatively low thermal conductivity, extracting heat from the center of the brush is substantially more difficult with an increased cross section. It is known to those familiar in the art that overall cooling time for all molding cycles for a multi-component brush can be minimized by balancing the size of each shot of plastic so that the brush is substantially uniformly divided by weight for each component, however this has the drawback of requiring a greater fraction of use of an expensive material, typically TPE, than would otherwise be required. In essence, both material use and capital equipment time cannot both be simultaneously optimized for this type of injection molded toothbrush.
Third—most thermoplastics shrink during cooling and solidification. Shrinkage can be mitigated by packing additional molten plastic into the center of the handle through the injection gate as the outer edges of the handle cool, however this mitigation loses effectiveness as the injection gate is placed away from the thickest portion of the handle and placement of the gate, which will have some tactile vestige, in the thick, gripping portion of the handle can lead to dissatisfaction during use. For many toothbrush handle designs, packing alone cannot mitigate the visible surface shrinkage and surface defects and internal defects associated with an increased handle cross section. These surface defects can be manifested as unintentional variations in surface gloss or texture, which contribute negatively to the look and feel of the part. Internal defects can be manifested as voids or bubbles inside the plastic, which can weaken the handle visibly or invisibly, depending on the degree of transparency of the plastic. It is known to those familiar in the art that a second component can be used to cover or hide negative cosmetic features such as gate vestiges or sink marks, however this cannot by nature work on the final shot which must necessarily have an uncovered gate vestige and may also contain sink marks in thick sections.
Fourth—the filling and packing of a thick-wall plastic parts in the injection mold cavity requires very high pressures, typically thousands of pounds per square inch, which necessitates mold cavities made from very high-strength materials, which are expensive and time-consuming to create. These extremely high pressures can in fact limit the speed of the manufacturing process by requiring complete or near-complete cooling and/or solidification of one plastic shot prior to injection of a subsequent shot.
Fifth—the injection of multiple shots of plastic in multiple steps necessarily requires each component of material to have at least one unique mold cavity portion which significantly adds to expense, complexity and difficulty in molding, especially where plastic and metal meet to form an edge, also known as a shutoff.
Sixth in multi-cavity production, the balance of fill between shots is especially difficult to control with TPEs, as they have a narrow range of processing temperatures and their viscosities do not vary substantially over this range.
In an attempt to overcome the difficulties associated with the use of injection molding to produce toothbrush handles having increased diameters, it has been suggested to produce toothbrush handles having a hollow body. For example, EP 0 668 140 or EP 0 721 832 disclose the use of air assist or gas assist technology to make toothbrushes having hollow, large cross-sectional handles. In the disclosed process, molten plastic is injected near the base of the toothbrush handle, wherein subsequently a hot needle is inserted into the molten plastic to blow gas into the molten plastic which is then expanded towards the walls of the injection mold. In a similar manner, U.S. Pat. No. 6,818,174 B2 suggests injecting a predetermined amount of molten plastic into the cavity to only partially fill the mold cavity and subsequently inject a gas through a gas injection port formed in the injection mold to force the molten plastic into contact with the walls of the mold cavity. Such injection molding processes using additional air injection have substantial difficulty forming hollow handle bodies with substantially uniform wall thickness, and as such, the potential for optimization of a handle for maximum ergonomic function in minimum material weight and manufacturing efficiency is limited. A further drawback to such injection molding processes in U.S. Pat. No. 6,818,174 B2 is the creation of a vent hole for the gas. EP 0 668 140 provides a possible solution to this problem via use of a moving injection pin to create a sealed part, however the integrity of this seal under the injection molding pressures created in the second shot is highly sensitive to processing conditions and is therefore not known. The vent hole is formed at the interface of molten plastic and high-pressure gas (and not by mold steel) and thus cannot be made predictably or with high precision. A still further drawback of hollow-handled toothbrushes made using gas-assist injection molding relates to the application or installation of a second, third or subsequent material to the toothbrush by injection molding, or overmolding, where the overmolded material may, in the process of sealing the necessary gas vent, intrude substantially into the hollow void created in the first gas injection step, as there is nothing to stop it besides friction and the near-atmospheric pressure inside the void. EP 0 721 832 illustrates this effect in detail. While this may still result in a cosmetically-acceptable part, it prevents use of shot-size-limiting devices such as valve gates and can add substantially to the cost of the part. Gas-assist injection molding does not substantially reduce injection pressure or melt energy required to form a plastic article. And as with all other known injection molding processes, multiple cavities and injection steps are required to add each material to the molded article.
A conventional method to create toothbrush handles having increased cross sections, such as electromechanical toothbrush handles, is to manufacture discrete parts of the handle separately using injection molding, then to assemble these parts in either a separate non-injection molding step, or in a subsequent injection molding step, or most often some combination of the two, whereby the discrete parts from the first step or steps are inserted into an injection mold first and one or more additional materials are injected around them, creating a hollow body from multiple parts. This manufacturing method still has the drawbacks of: requiring the complete melting of plastic at every molding step, high pressures at every molding step, associated equipment involved with injection molding at every molding step, and in addition may have added labor expense associated with both in-mold and out-of-mold assembly of discretely-molded parts, plus the added expense and inconvenience of multiple steel cavity sets per part manufactured. The use of injection molding to create multiple discrete parts has also the disadvantage that each part must not contain any substantial undercut from which the mold core forming a concave surface of the injection-molded part could not be extracted from the part after molding; or in the case where such undercut exists, it must be created carefully by means such as collapsing mold cores and is thus subject to extensive constraints on the surrounding geometry. Further, mold cores must typically contain some mechanism to cool or remove heat, and would thus be difficult or impossible to create to make internal geometry for most manual toothbrushes which may have diameters of 10 mm and lengths beyond 150 mm. The lack of undercuts in discrete parts combined with the length and diameter of cores required to make non-undercut handle parts combined with the desire for multiple areas of variation in cross sectional area on a toothbrush handle would thus require any discretely-assembled handles to have multiple mating surfaces which would preferably require seals to maintain barriers to moisture and debris, even under time and repeated use. To eliminate the need for gaskets and expensive, pliant materials, these seals are typically made using permanent-fastening operations such as ultrasonic welding or gluing.
Installation of soft-touch or other second materials to hollow molded articles can be made by non-injection molding means such as welding, gluing or use of flexibility of the soft-touch material to itself grip an undercut pre-molded into the main article. These methods all have disadvantages however in long-term adhesion, especially to thermoplastics with less-active surfaces made from materials such as polypropylenes. Durable articles made from multiple components which must be used in unpredictable circumstances and environments such as consumers' bathrooms must necessarily be constructed more robustly than for example, disposable articles or packages.
Electromechanical toothbrushes in particular are susceptible to problems of assembly, as they are necessarily hollow in order to include batteries, motors and associated electrical linkages and drive components which must be all placed inside with some degree of precision. To avoid the problems and expense of welding plastic parts together and multiple assembly steps of a sealed outer shell, it has been proposed to blow mold the handle for electromechanical toothbrushes. In the assembly of a blow molded electromechanical toothbrush it is necessary to leave the blow molded portion of the handle open in at least one end to accommodate the motor, batteries, and drive system components. In this process, the minimum diameter of at least one opening to the blow molded handle must exceed the smallest linear dimension of every component that will be inserted. Such a large opening would be a drawback in a non-electromechanical handle, which has no need to accommodate internal component entry, and would necessitate an overly-large second part or cap to prevent intrusion and collection of water, paste, saliva and other detritus of conventional use. Such an overly-large opening, if positioned near the head, would interfere substantially with ergonomic use of the brush. Additional constraints to the geometry on the inside surface of the cavity, for example to locate motors, housings, batteries, etc. which must be positioned inside accurately as to be rigidly fixed will also be detrimental to the overall blow molding process, as the majority of the inner cavity surface of a blow molded part cannot be defined directly by steel in the mold surfaces, and is instead defined indirectly by steel on the outer surface of the handle combined with the wall thickness of the parison, blowing pressure and stretch ratio of the final part to the original parison or preform thickness. Such constraints of these process variables will necessarily limit manufacturing efficiencies.
To accommodate activation of electrical components via a standard button or mechanical switch, at least some portion of a blow molded electromechanical toothbrush handle should be made thin enough to flex substantially under pressure of a finger or hand squeeze. Such a thin-walled structure or film-walled structure necessarily requires some strengthening mechanism to ensure durability and rigidity under use. An internal frame or cap, as described in WO 2004/077996 can be used to provide this necessary strengthening mechanism in an electromechanical toothbrush, but would be a drawback to a manual brush, which must contain no additional components to function adequately, in extra expense, complexity and additional load-bearing parts. Further, due to the linear nature of the motor, power source, and drive shaft of electromechanical toothbrushes there are no or minimal variations to the cross-sectional area of the inner cavity; such that the inner cavity walls provide mechanical support to the internal components to reduce or eliminate unwanted movement or shifting. Alternately, it would be required to cut or drill a hole in the blow molded part and then to fasten somehow a flexible cover to transmit the mechanical motion from the outside of the brush to the switch inside.
An electromechanical toothbrush handle, made by blow molding or injection molding, is typically manufactured with an opening at both ends: At a distal end there is typically an opening to accommodate the mechanical translation of power through a drive mechanism to the toothbrush head, and at a proximal end there is typically an opening to accommodate insertion of components during manufacturing and possibly also insertion or removal of the battery by the user. Such a second opening would be unnecessary for a manual toothbrush and would create drawbacks in the need for additional seals and mechanical fasteners. In some blow molding processes, the formation of openings at the distal and proximal ends of the molded part are intrinsic to the process and would benefit the formation of a double-open-end handle, but would not be necessary for a manual toothbrush handle.
To reduce weight while maintaining stiffness, some toothbrush handles are made from bamboo or balsa wood, however these materials have disadvantages in that they are not easily formable into complex three-dimensional shapes which can be comfortably gripped. Further, these materials are anisotropic, meaning they have elastic moduli and yield strengths or ultimate strengths which vary with the direction of applied load. Carbon-fiber composites and glass-filled injection-molded plastics are other common examples of anisotropic materials which could be used to make lighter and stronger toothbrushes. Articles made from these materials must therefore be formed with their strongest axis or ‘grain’ aligned substantially with the major axis of the article in order to resist fracture during the bending forces common to use. Both carbon fiber and glass-filled thermoplastic composites also tend to fail in a brittle manner, with little ductility. This type of failure is undesirable in a device that is placed in the mouth: More desirable is a device which, when subjected to loads substantially greater than their design loads, fail first in some permanent bending mode versus a sudden fracture. Further, these materials do not contain intrinsically all of the properties necessary to create light weight, strength in bending and soft-touch, high-friction grip. This creates an extra necessary step in the preparation of the material prior to forming or machining. This alignment of the grain also can present a specific disadvantage to woods in general in that the presentation of splinters of material is most likely to occur in the direction aligned to typical forces applied by the hand during brushing.
To make a toothbrush without relying on anisotropic materials such as woods, the articles could be made lighter through the use of non-homogeneous but isotropic materials, such as foamed plastics. Foamed plastics present an advantage in that they can offer a higher strength-to-weight ratio than solid plastics without regard to material orientation. The overall weight savings possible with foamed plastics may be limited however, as the bubbles inside the plastic which create the weight savings also create stress concentrations which will severely reduce strength in tension and can reduce ductility. While foamed plastics can provide substantial strength in compression (and are used for exactly this purpose in applications such as packing materials where material weight combined with resistance to compressive crushing is a critical issue) the weakness in tension severely affects bending strength and prevents uniformly-foamed plastics from serving as load-bearing elements in articles which must maintain strength and stiffness in bending during normal use.
It is familiar to those in the art to use extrusion blow molding to create single-component or single-material lightweight hand-held articles, such as children's toys, plastic bats, plastic golf clubs or any large, plastic article which benefits from being lighter in weight. While these articles can be both stiff and strong in bending, they also generally contain drawbacks which would limit their general use in semi-durable, Class-I medical devices, such as toothbrushes. First, such articles typically contain significant flash along parting lines, or in any locations where the parison is larger in cross sectional area than is the cavity to which it is blown. In these locations the parison folds within the cavity and substantial flash is created, even in the absence of cavity parting line. Second, most articles contain some significant vestige of blowing in the form of a hole, which may be accurately or inaccurately formed. Such a vestige would be regarded as a significant defect in a Class-I medical device which must prohibit breach or entry of contaminants to a hollow interior which does not drain effectively. Third, the relative size of these articles is large in comparison to the size of these defects, and the overall function of the articles is not severely affected by these defects. In many cases, the size of the article itself renders the manufacturing process easier, with respect to the minimization of defects. It is not challenging to extrusion blow mold articles, packages or bottles in the size range common to manual toothbrush handles—if the plastic wall thickness can be minimized in proportion to the overall cross section. Such articles exist in the form of small, typically squeezable, tubes or bottles which in fact benefit from having a very thin, deformable wall which enables dispensing of internal contents, making them unusable or significantly inferior as toothbrushes.
Extrusion and injection-blow-molded handles for single-component semi-durable consumer goods such as feather dusters and tape dispensers are also known, but again these articles would not meet criteria for semi-durable Class I medical devices, specifically with regard to the sealing of the necessary blowing orifice against intrusion of water or other contamination, and in the case of extrusion blow molding, in the appearance of flash on the articles in areas that would directly contact or go into the mouth. These articles can also be brittle, and when too much force is applied, can break or snap suddenly and without ductility, producing sharp edges, making them unusable for use in the oral cavity.
Multi-material blow molded packages, such as water bottles, are known to those familiar in the art. In these embodiments, smooth blow molded bottles are provided with tactile, high-friction surfaces via the use of an in-mold labeling technique, whereby previously injection-molded, textured labels are placed into mold cavities prior to introduction and blowing of the semi-molten parison of extruded plastic. While these articles do provide the advantage of a large gripping surface which is improved by addition of a high-friction textured surface, they are by nature highly-deformable or squeezable packages designed for liquid storage and dispensing and would serve poorly as toothbrushes as there is no obvious method to attach bristle tufts without injection molding. Further, the injection molding of the soft-grip labels requires an additional set of mold cavities and a molding step separate from the blow molding step.
It has also been proposed to manufacture hollow toothbrushes, however no existing disclosure in the prior art addresses the issues of: Strength in bending, stiffness in bending, overall rigidity, mitigation of flash or other sharp defects, variations in cross-sectional area, and obstruction or sealing of the blow hole vestige. Any one of these defects in a blow molded toothbrush or toothbrush handle would severely affect the utility of the article, and as such, improvements are needed to enable a hollow article with material savings maximized by uniform wall thickness which is suitably strong and stiff in bending without breaking in use and does not leak or present uncomfortable defects to the user.
In view of these disadvantages reflected in the prior art, it is an objective of the present invention to provide an improved method for producing a toothbrush handle, which avoids the drawbacks of the prior art.